Field of the Invention
The present invention concerns a method to determine a control sequence for operating a magnetic resonance imaging apparatus to generate image data of the inside of an examination subject, as well as a control sequence determination device, and a magnetic resonance imaging apparatus with such a control sequence determination device, to implement such a method.
Description of the Prior Art
In a magnetic resonance system (also called a magnetic resonance tomography system or magnetic resonance imaging system), the body to be examined is typically exposed in a scanner to a relatively high basic magnetic field, for example of 1, 5, 3 or 7 Tesla, with the use of a basic field magnet system. A magnetic field gradient is additionally applied with the aid of a gradient system. Radio-frequency excitation signals (RF signals) are then emitted by suitable antenna devices via a radio-frequency transmission system, which cause nuclear spins of specific atoms to be excited to resonance by being deflected, by an amount known as a defined “flip angle”, relative to the magnetic field lines of the basic magnetic field. Upon relaxation of the nuclear spins, radio-frequency signals (magnetic resonance signals) are radiated that are received by suitable reception antennas and then are processed further. The desired image data are reconstructed from the raw data acquired in such a manner.
For a defined measurement, a defined pulse sequence is emitted from a control unit (control computer) of the apparatus, which is composed of a series of radio-frequency (RF) pulses (in particular excitation pulses and refocusing pulses) as well as gradient pulses to be emitted in coordination with the RF pulses in different spatial directions, as well as readout windows set to match these during which the induced magnetic resonance signals are received. The gradient pulses are defined by their gradient amplitude, the gradient pulse duration and their edge steepness dG/dt, typically designated as a “slew rate”. Another important gradient pulse variable is the gradient pulse moment, which is defined by the integral of the amplitude over time.
The timing within the sequence (i.e. in which time intervals which pulses follow one another) is thereby significant for imaging. A number of control parameters is normally defined as a measurement protocol, which has been created in advance and can be retrieved (from a memory, for example) for a specific measurement. The retrieved protocol can, if necessary, be modified on site by the operator, who can provide additional control parameters such as, for example, a defined slice interval of a stack of slices to be measured, a slice thickness, etc. A pulse sequence (is also designated as a control sequence) is then calculated on the basis of all of these control parameters.
During a pulse sequence, switching takes place frequently between the magnetic gradient coils via which the gradient pulses are emitted. Eddy currents that are thereby generated—in particular in other components of the magnetic resonance scanner, are one reason for the known development of noise during the switching of the gradients. In particular, a high edge steepness of the gradient pulses contributes to the noise exposure. In addition to this, steep edges lead to a higher energy consumption and additionally place higher demands on the gradient system. The rapidly changing gradient fields lead to distortions and fluctuations in the gradient coils, and to the transfer of these energies to the scanner housing.
Various solutions in the design of hardware in order to reduce the noise exposure have been proposed, for example by potting or vacuum sealing the gradient coils. Another possibility is to pay attention to the gradient curve in advance, in the calculation of the pulse sequences. In practice, there are therefore apparatuses that offer differing manners of operation known as “gradient modes”. For example, the operator can hereby select between a normal mode and a particularly quiet gradient mode as needed. In the quiet gradient mode, a maximum allowable edge steepness for the gradient pulses is set to a lower value, which leads to the situation that the measurement is quieter than in the normal mode. However, normally this setting disadvantageously not only leads to a longer measurement time overall, but also has the effect that the image quality (for example the contrast and/or the resolution) is reduced. Given such a limitation of the global maximum slew rate, a compromise must always be found between the reduction of the noise volume, the measurement time and the image quality. For example, a longer echo spacing—thus a greater interval between the echoes—has a negative effect on the contrast and the image sharpness in a control sequence in which a series of echo signals is induced, for example in spin echo sequences (SE sequences) or in turbo spin echo sequences (TSE sequences).
In a number of pulse sequences that are often used in clinical magnetic resonance tomography (MRT), for example, the cited echo sequences, pulses known as gradient spoiler pulses (spoilers, for short) are activated in addition to the gradient pulses that are necessary for a spatial coding. Gradient spoiler pulses (which, in some cases, particularly if they occur in pairs, are also called gradient crusher pulses, crushers for short) are produced by the same gradient coils immediately before and/or after the spatial coding gradient pulses, and ensure that (for example) unwanted free induction decay (FID) signals are suppressed. The spoilers or crushers must have a defined spoiler or crusher moment so that they suppress the FID signals with certainty.
A large part of the noise in MRT examinations results from the spoiler pulses, in particular in the use of SE or TSE sequences.
In order to alleviate the problems noted above, a given quiet pulse sequence is disclosed, in German Patent Application DE 10 2012 219 010, wherein an optimization method is executed that optimizes the edge steepness of the crushers in combination with the pulse level and duration of the gradient pulses bounded by the respective crusher.
For example, one result of this optimization can then be a pulse train as shown in FIG. 1, which can be used as input data in the method according to the invention as described later herein.
The input data diagrammed in FIG. 1 include a control sequence AS with a schematically shown portion of a sequence diagram of a radio-frequency pulse train RFP that includes refocusing pulses RF1, RF2, and a gradient pulse train GP that includes slice selection gradient pulses SLP1, SLP2. The slice selection gradient pulse thereby serves as a spatial selection pulse for selection of a spatial region, i.e. in particular for selection of the slice in the examination subject that the refocusing should affect. The slice selection gradient pulse is executed simultaneously with the refocusing pulse so that this acts only on the selected slice or, respectively, the selected spatial region.
As an example, in the following only the slice selection gradient pulse SLP1 is considered. The slice selection pulse SLP1 is bounded by spoiler pulses SP1, SP2 that together form a crusher pulse and have an edge steepness that is optimal with regard to noise development. The refocusing pulse RF1 that is emitted to chronologically match the slice selection gradient pulse SLP1 is emitted with a high amplitude A1 (maximum value of the amplitude). This high amplitude has the effect that high demands are placed on the hardware of the magnetic resonance system that is used, and also causes the SAR exposure value of an examination subject scanned with this pulse sequence to increase discontinuously. Both are not always acceptable, in particularly in clinical operation.